Migration of Birds

Techniques for Studying Migration

Before we discuss the many intricacies of how, when, and where birds
migrate, one should have a general idea of how migration data are collected
and what methods are currently being used to increase our knowledge. Since
this publication first appeared in 1935, many new procedures have been
used in the study of bird migration. One of these, radar, has been an invaluable
adaptation of a technique developed for a quite different, but related,
purpose.

Direct ObservationThe oldest, simplest, and most frequently used method of studying migration
is by direct observation. Size, color, song, and flight of different species
all aid the amateur as well as the professional in determining when birds
are migrating. Studies begun by Wells W. Cooke and his collaborators (Cooke
1888-1915) and continued by his successors in the U.S. Bureau of Biological
Survey (later U.S. Fish and Wildlife Service) were of particular importance
in the earlier years of these investigations in North America. Some of
the largest and most interesting routes and patterns were sorted out by
tediously compiling and comparing literally thousands of oberservations
on whether a species was or was not seen in a given locality at a particular
time of the year. More recently, "The Changing Seasons" reports by many
amateur bird observers in Audubon Field Notes (now American Birds)
have been a most important source of information on direct observation
of migration. In the agregate, direct observation has contributed much
to our knowledge of migration, but, as will be pointed out in other sections,
until a few years ago, observers were not aware of some of the biases in
this technique.

The "moon watch" is a modification of the direct observation method.
It has long been known that many species of birds migrate at night. Until
recently, it was not apparent just how important nocturnal migration really
is. Significant information has been derived from watching, through telescopes,
the passage of migrating birds across the face of a full moon. Since the
actual percent of the sky observed by looking through a telescope at the
moon is extremely small (approximately one-hundred thousandth of the observable
sky), the volume of birds recorded is small. On a night of heavy migration,
about 30 birds per hour can be seen. The fact that any birds are observed
at all is testimony to the tremendous numbers passing overhead. Large-scale,
cooperative moon-watching studies have been organized and interpreted by
George H. Lowery, Jr. (1951; Lowery and Newman 1966).

Another specialized direct observation approach which has yielded
important information on the spatial and altitudinal distribution of night
migrating birds has been the use of small aircraft equipped with auxiliary
landing lights (Bellrose 1971). Major disadvantages of night observation
are that species cannot be identified and that birds continue to migrate
without a full moon. However, these techniques do give information on the
nocturnal migration movements that could not be obtained by other methods.

AuralAn adjunct to the previously described nocturnal observation methods,
which has potential for species identification, is the use of a parabolic
reflector with attached microphone to amplify call (chip) notes (Ball 1952;
Graber and Cochrane 1959). This device, when equipped with a tape recorder,
can record night migrants up to 11,000 feet on nights with or without a
full moon. A primary disadvantage is that one cannot tell the direction
a bird is traveling and there is considerable difficulty in identifying
the chip notes made by night migrants. In addition, the bird may not call
when it is directly over the reflector and consequently it would not be
recorded. These calls are quite different from the notes we hear given
by familiar birds during the daytime while they are scolding an intruder
or advertising their territory.

Preserved SpecimensReference material consisting of preserved bird skins with data on
time and place of collection exist in many natural history museums. The
essential ingredient in studying migration by this method is to have an
adequate series of specimens taken during the breeding season so differences
in appearance between geographically separated breeding populations of
the same species can be determined. Such properly identified breeding specimens
may be used for comparison with individuals collected during migration
to associate them with their breeding areas (Aldrich 1952; Aldrich, Duvall,
and Gels 1958). This supplies a convenient way of recognizing and referring
to individuals representative of known populations wherever they may be
encountered.

MarkingIf birds can be captured, marked, and released unharmed, a great deal
of information can be learned about their movements. Many different marking
methods have been developed to identify particular individuals when they
are observed or recaptured at a later date. A few of the general methods
are summarized in this section.

Bands, Collars, Streamers

Since 1920, the marking of birds with numbered leg bands in North America
has been under the direction of the U.S. Fish and Wildlife Service in cooperation
with the Canadian Wildlife Service. Every year professional biologists
and voluntary cooperators, working under permit, place bands on thousands
of birds, game and nongame, large and small, migratory and nonmigratory,
with each band carrying a serial number and the legend, NOTIFY FISH AND
WILDLIFE SERVICE, WASHINGTON, D.C., or on the smaller sizes, an abbreviation.
When a banded bird is reported from a second locality, a definite fact
relative to its movements becomes known, and a study of many such cases
develops more and more complete knowledge of the details of migration.

The records of banded birds are also yielding other pertinent
information relative to their migrations such as arrival and departure
dates, the length of time different birds pause on their migratory journeys
to feed and rest, the relation between weather conditions and starting
times for migration, the rates of travel for individual birds, the degree
of regularity with which individual birds return to the summer or winter
quarters used in former years, and many other details. Many banding stations
are operated systematically throughout the year and supply much information
concerning the movements of migratory birds that heretofore could only
be surmised. The most informative banding studies are those where particular
populations of birds are purposely banded to produce certain types of information
when they are recovered. Examples of such planned banding are the extensive
marking of specific populations of ducks and geese on their breeding grounds
by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service,
as well as in "Operation Recovery," the cooperative program of banding
small landbirds along the Atlantic Coast (Baird et al. 1958). When these
banded birds are recovered, information concerning movements of specific
populations or the vulnerability to hunting is gained. Colored leg bands,
neck collars, or streamers can be used to identify populations or specific
individuals, and birds marked with easily observed tags can be studied
without having to kill or recapture individuals, thus making it a particularly
useful technique.

We have learned about the migratory habits of some species through
banding, but the method does have shortcomings. If one wishes to study
the migration of a particular species through banding, the band must be
encountered again at some later date. If the species is hunted, such as
ducks or geese, the number of returns per 100 birds banded is considerably
greater than if one must rely on a bird being retrapped, found dead, etc.
For example, in mallards banded throughout North America the average number
of bands returned the first year is about 12 percent. In most species that
are not hunted, less than 1 percent of the bands are ever seen again.

In 1935, Lincoln commented that, with enough banding, some of
the winter ranges and migration routes of more poorly understood species
would become better known. A case in point is the chimney swift, a common
bird in the eastern United States. This is a nonhunted species that winters
in South America. Over 500,000 chimney swifts have been banded, but only
21 have been recovered outside the United States (13 from Peru, 1 from
Haiti, and the rest from Mexico). The conclusion is simply this: Whereas
banding is very useful for securing certain information, the volume of
birds that need to be banded to obtain a meaningful number of recoveries
for determining migratory pathways or unknown breeding or wintering areas
may be prohibitive. One problem in interpretation of all banding results
is the fact that recoveries often reflect the distribution of people rather
than migration pathways of the birds.

Other methods used to mark individuals in migration studies include
clipping the tip end off a feather (not a major flight feather) with a
fingernail clipper or touching the feather with colored paint or dye. This
marking technique is obviously good for only as long as the bird retains
the feather (usually less than one year), but allows the investigator to
recognize whether the bird has been handled previously or not.

Radio TrackingOne of the most promising methods of tracking the movements of individual
birds in migration has been developed in recent years. It is called radio
tracking, or telemetry, and consists of attaching to a migrating bird a
small radio transmitter that gives off periodic signals or "beeps". With
a radio receiving set mounted on a truck or airplane, it is possible to
follow these radio signals and trace the progress of the migrating bird.
One of the most dramatic examples of this technique was reported by Graber
in 1965. He captured a grey-cheeked thrush on the University of Illinois
campus and attached a 2.5-gram transmitter to it (a penny weighs 3 grams).
The bird was followed successfully for over 8 hours on a course straight
across Chicago and up Lake Michigan on a continuous flight of nearly 400
miles at an average speed of 50 mph (there was a 27 mph tail wind aiding
the bird). It is interesting to note that while the little thrush flew
up the middle of Lake Michigan, the pursuing aircraft skirted the edge
of the lake and terminated tracking at the northern end after running low
on fuel while the bird flew on. The limitations of radio telemetry, of
course, are the size of the transmitter that can be placed on birds without
interfering with flight and the ability of the receiving vehicle to keep
close enough to the flying bird to detect the signals. Despite this difficulty
there has been considerable development in the technology, and encouraging
results to date give promise for the future, particularly when receivers
can be mounted on orbiting satellites (Graber 1965; Bray and Corner 1972;
Southern 1965).

Radar ObservationsOne of the developments of our modern age of electronics has been the
discovery that migrating birds show up on radar screens used in monitoring
aircraft. At first. the screen images caused by flying birds were a mystery
to radar operators, and they designated the dots "angels." Later when their
nature was understood, students of bird migration seized on the unique
opportunity to obtain information on movements of birds over extensive
areas (Sutter 1957; Drury 1960; Lacke 1963a, b; Bellrose 1967; Graber 1968;
and Gauthreaux 1972a, b).

Three types of radar have been used for studing birds: 1) general
surveillance radar, similar to ones located at airports, that scans a large
area and indicates the general time and direction of broad movements of
birds; 2) a tracking radar that records the path of an airplane (or bird)
across the sky by "locking on" to a designated "target" and continuously
following only that object; and 3) a Doppler radar similar to those operated
by law enforcement agencies for measuring the speed of a passing automobile.
The latter radar set is useful in determining the speed of flying birds.

The use of radar in migration studies has been invaluable in determining
direction of mass movement, dates and times of departure, height of travel,
and general volume, especially at night. One interesting fact to come out
of current radar work is the discovery of relatively large movements of
warblers and other land birds migrating over the seas rather than along
the coastlines and in directions observers were completely unaware of a
few years ago.

Laboratory

Orientation and Navigation

Studies on how migrating birds orient (travel in one compass direction)
or navigate (travel toward a specific goal) have received increasing emphasis
in the past 20 years. These studies have focused on the ability of birds
to orient themselves by the position of the sun and stars. Outstanding
in this facet of research have been the works of Matthews (1951, 1955),
Kramer (1952, 1959, and 1961), Sauer and Sauer (1960), Mewaldt and Rose
(1960), Sauer (1961), Hamilton (1962a, b), Schmidt-Koenig (1963, 1964),
and Emlen (1969). The basic method used in the experiments is to observe
the direction in which confined birds attempt to move during the period
of migratory restlessness. The birds are not permitted to have any view
of the landscape but only the sky above them. In some cases the positions
of the celestial bodies are changed by the use of mirrors to see the effect
on the orientation of the experimental birds. In other cases the experiments
are performed in plantetariums so positions of the stars in the artificial
heavens can be manipulated and the effect observed.

Physiology of Migration

The physiological basis for bird migration has received considerable
attention, particularly the effects of seasonal increases and decreases
in daylight and the seasonal rhythms occurring within animals and referred
to as "biological clocks." Investigations in this field include the pioneering
work on the relationship of photoperiod (daylength) to migration by Rowan
(1925, 1926) and many subsequent studies (Wolfson 1940, 1945; Marshall
1961; King, Barker and Farner 1963; King and Farner 1963; King 1963; Farner
1955, 1960; and Farner and Mewaldt 1953). These studies have become ever
more deeply involved in the intricate relationships between photoperiod,
endocrine interactions, gonad development, fat deposition, and migratory
unrest. They add to our knowledge of the mechanisms that regulate the migratory
behavior we observe.